1. Field of the Invention
The present invention relates to a method for forming a gasket closure for containers and more particularly to a method for forming vinyl chloride plastisol gaskets in closures formed from synthetic plastic resins.
2. The Prior Act
With the advent of commercially available synthetic plastics, i.e. easily formable thermoplastic synthetic resins, it has become common practice to form various products therefrom due to inexpensive material and production costs. Some of such products include container closures. Heretofore, container closures molded from polyolefin resins such as polyethylene and polypropylene, have incorporated a positive locking means thereon, such as screw thread or a snap lock, or likewise, coordinated with an associated container solely through a friction fit. Although these plastic closures have found wide application in the packaging field, they have been found deficient in sealing integrity for containers used for the packaging of liquids and fine powders. Such applications require the insertion of a sealing gasket or a prefabricated composite liner to provide a leak-proof seal between the closure and the container to which it is fitted. Gaskets have not been generally used in plastic closures because the thermoplastic resin such as a polyolefin resin from which the closure is molded is generally not compatible with conventional methods for forming a gasket, as such gasket forming methods employ temperatures for forming the gasket at which the olefin plastic closure undergoes softening, stress relaxation and warpage. For example, in the widely used "spin-lining method" for forming gaskets in closures, a vinyl chloride polymer based plastisol composition in an uncured, paste-like condition is dispensed from one or more nozzles into metal closure shells which are inverted on a chuck rotating at high speed. Due to centrifugal force, the plastisol compound assumes the desired configuration and shape. After being thus deposited or "flowed-in", the plastisol compound is fused (fluxed) by baking the closure shell in an oven at temperatures in the order of 160.degree.-200.degree. C. from 1/2 to 5 minutes, or the "flowed-in" compound may be molded and fused in the closure shell with hot molding punches and platens in a turret arrangement. The fact that polyolefin resins such as polyethylene and polypropylene have softening points of 90.degree. C. and 150.degree. C. respectively makes them poor candidate materials for closures to be spun-lined with plastisol compounds and baked at 160.degree. C.
Other methods considered for the delivery of elastomeric gasket material to polyolefin closures include hot melt application equipment. The generally low softening temperatures, e.g., 80.degree.-120.degree. C. makes these hot melt materials substantially useless in providing hermetic seals in pasteurization and sterilizalion processes which require temperatures in the order of 70.degree.-125.degree. C.
There is therefore a need in the plastic closure art for a method whereby liquid plastisol compounds can be deposited at high speeds and uniform thickness in plastic closures, e.g., using spin-lining equipment, and then fluxed within the closure without damage to the dimensional and physical properties of the closure. The resultant closures would be expected to withstand sterilization and pasteurization processes.
The prior art has attempted to form sealing gaskets fabricated from vinyl chloride resin based plastisols in plastic closures. For example, U.S. Pat. No. 4,304,744 discloses the various unsuccessful attempts made by the art to overcome the problem of heat distortion of plastic closures when it is attempted to flux the vinyl chloride resin based plastisol deposited in the closure. Included in these unsuccessful attempts to solve the closure heat distortion problem was the use of alternative heating means for fusion of the plastisol, namely, induction heating (British Pat. No. 818,418) and microwave heating (British Pat. No. 1,327,583). The disadvantages of these alternative heating means are discussed at length in U.S. Pat. No. 4,304,704 and need no further discussion here.
The invention disclosed in U.S. Pat. No. 4,304,704 attempts to solve the plastic closure heat distortion problem wherein the plastic closure having deposited therein unfluxed plastisol composition is preheated by conventional means to about 5.degree.-35.degree. C. below the melting point of the plastic material (e.g., for polypropylene this is about 160.degree. C.) which heating is followed by microwave heating (e.g., at 300-300,000 megahertz) to flux the plastisol. Unfortunately, it has been found that the preheating step is sufficient to cause softening, distortion and dimensional changes to the closures fabricated from plastic materials such as polyolefins such as polyethylene and polypropylene.